3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
A micro cell, a femto cell, and a pico cell, etc., having a small service area can be installed in a specific location of a macro cell having a wide coverage.
Due to mobility of a user equipment (UE) represented as a mobile device, quality of a currently provided service may deteriorate or a cell capable of providing a better service may be detected. Accordingly, the UE may move to a new cell, which is called performing of the mobility of the UE.
Circuit Switched fallback (CSFB) is a standardized solution that delivers voice and SMS over existing 2G/3G circuit switched network. The CSFB solution was widely supported by many operator as it requires minimum changes on legacy 2G/3G networks, and the CSFB is well suitable for the initial use cases of LTE network where LTE is used for data service only.
Upon a CS service being triggered, the CSFB requires the UE to make a Radio Resource Control (RRC) connection with LTE if the UE is in a RRC_IDLE state rather than directly selecting a cell of CS RAT while in RRC_IDLE state. These procedures occur unnecessary delay, then a quality of service may be deteriorated. Therefore, it is requested to suppose a method for communication allowing the UE to initiate the CS service more quickly.